1. Field of the Invention
The invention relates to magnetic resonance imaging system which includes: an interventional instrument which is provided with an indicator element for influencing a magnetic resonance image. The invention also relates to an interventional instrument which is intended to co-operate with a magnetic resonance imaging system and is provided with an indicator element for influencing a magnetic resonance image.
2. Description of Related Art
A magnetic resonance imaging system of this kind is used notably for a medical interventional procedure during which the interventional instrument is introduced into the body of the patient to be examined. During the introduction of the interventional instrument one or more magnetic resonance images are made of the patient to be examined. Because the indicator element influences the magnetic resonance image, the position of the interventional instrument can be picked up and the instantaneous position of the interventional instrument can also be reproduced in a rendition of the magnetic resonance image. The indicator element preferably induces a local disturbance in the magnetic resonance image. On the basis of the magnetic resonance images the interventional instrument can thus be tracked inside the body, even though it is obscured from direct view.
An interventional instrument of this kind is known from United States patent U.S. Pat. No. 5,728,079. The cited patent also mentions a magnetic resonance imaging system provided with such an interventional instrument.
The known interventional instrument is a catheter provided with a hollow tubular holder and the indicator element is a concentric layer of a paramagnetic material. The concentric paramagnetic layer is provided in the form of a cylindrical sheath whose longitudinal axis is coincident with the longitudinal axis of the holder. The paramagnetic material influences the magnetic resonance image of a patient to be examined by means of the magnetic resonance imaging system. The influencing of the magnetic resonance image makes it possible to derive the position of the interventional instrument within the body of the patient without the instrument being directly visible. The influencing of the magnetic resonance image by the paramagnetic layer has an adverse effect on the diagnostic quality of the magnetic resonance image. Due to the influencing by the indicator element, notably anatomical details in the vicinity of the position of the interventional instrument are reproduced in a disturbed manner.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant""s invention of the invention subsequently claimed.
It is an object of the invention to provide a magnetic resonance imaging system with an interventional instrument in which the position of the interventional instrument within the body of the patient to be examined can be more accurately derived by influencing the magnetic resonance image and/or in which the magnetic resonance image is disturbed less than by the known interventional instrument. It is notably an object of the invention to provide a magnetic resonance imaging system with an interventional instrument whereby the position of the interventional instrument can be accurately derived without seriously affecting the diagnostic quality of the magnetic resonance image.
This object is achieved by means of a magnetic resonance imaging system according to the invention which is characterized in that the degree of influencing of the magnetic resonance image is adjustable.
The magnetic resonance imaging system according to the invention is particularly suitable for use during an intervention. During an intervention the interventional instrument, for example, a catheter, is introduced into the body of the patient and magnetic resonance images of the body of the patient to be examined are then formed, the interventional instrument being reproduced therein. The influencing of the magnetic resonance image represents the position of the interventional instrument; this enables the position of the interventional instrument to be derived from the influencing of the magnetic resonance image. Notably, the small region in the magnetic resonance image which is influenced corresponds with the position of the interventinal instrument.
Because the influencing of the magnetic resonance images by the interventional instrument is adjustable, on the one hand the magnetic resonance images can be strongly influenced so that the position of the interventional instrument can be clearly observed in the magnetic resonance images. On the other hand, through very weak influencing of the magnetic resonance images by the interventional instrument it can be ensured that the diagnostic quality of the magnetic resonance images is hardly degraded by the interventional instrument. In the case of very weak influencing, the magnetic resonance image accurately reproduces the anatomy of the patient to be examined. As a result, the position of the interventional instrument within the body of the patient can be accurately derived. For example, magnetic resonance images are formed during the introduction of the interventional instrument, the interventional instrument then influencing the magnetic resonance images to a substantial degree so that the interventional instrument can be readily tracked in the magnetic resonance images. As a result, the interventional instrument is clearly reproduced in the magnetic resonance image so that the interventional instrument can be tracked within the body of the patient without being directly visible. The interventional instrument can thus be safely guided through the body of the patient without unnecessary damaging of tissue. Once the interventional instrument has been guided to the desired position, the degree of influencing of the magnetic resonance images is reduced as much as possible. Magnetic resonance images in which hardly any disturbances occur can then be formed of the body of the patient. Such magnetic resonance images have a high diagnostic quality because they reproduce the anatomy of the patient to be examined with many details and exactly. When subsequently the interventional instrument is moved again, the degree of influencing of the magnetic resonance images by the interventional instrument is adjusted to a higher value again, so that the interventional instrument can be suitably tracked in the magnetic resonance images. The degree of influencing of the magnetic resonance images can notably be adjusted in such a manner that anatomical details are reproduced in the magnetic resonance image with only few disturbances, the disturbance nevertheless being sufficient for accurate determination of the position of the interventional instrument.
The indicator element causes the influencing of the magnetic resonance image. Such influencing is produced by the magnetic properties of the indicator element. Specifically, the magnetic susceptibility of the indicator element deviates essentially or even considerably from the magnetic susceptibility of other parts, such as the holder, of the interventional instrument. Furthermore, the magnetic susceptibility of the indicator element preferably deviates significantly from the magnetic susceptibility of the tissue in the vicinity of the indicator element. It is thus achieved that the indicator element can induce a distinct disturbance in the magnetic resonance image, so that the indicator element can be clearly reproduced in relation to the anatomical structures in the magnetic resonance image. Preferably, the degree of influencing of the magnetic resonance image by the interventional instrument is adjustable by adjustment of the orientation of the indicator element relative to the steady magnetic field of the magnetic resonance imaging system. Generally speaking, the orientation of the indicator element can be readily controlled without influencing the interventional functions of the interventional instrument. This is because the interventional instrument is usually shaped as an elongate holder which can be rotated about its longitudinal axis within the body within the body of the patient to be examined. When the holder is rotated about its longitudinal axis, the indicator element is also rotated relative to the steady magnetic field. Evidently, it is also possible to construct the interventional instrument in such a manner that the indicator element can be rotated, inside the holder, relative to the steady magnetic field, without the holder being rotated. The interventional instrument is then moved within the body of the patient while the elongate holder has an orientation relative to the steady magnetic field such that considerable influencing of the magnetic resonance images occurs. Attractive results are achieved notably when the longitudinal axis of the holder encloses a significant angle relative to the direction of the steady magnetic field. In the vast majority of interventional procedures the interventional instrument is indeed used in such a manner that the longitudinal axis of the instrument is not directed parallel to the direction of the steady magnetic field.
With the exception of the indicator element of the interventional instrument, the components, such as the holder, preferably have a magnetic susceptibility which does not influence or only hardly influences the magnetic resonance image, whereas the indicator element has a magnetic susceptibility which is capable of strongly influencing the magnetic resonance image. It has been found that the interventional instrument can be clearly and accurately tracked in the magnetic resonance images when use is made of such an indicator element.
The indicator element is constructed, for example as a strip which extends in a direction transversely to the longitudinal axis of the holder. By rotation of the interventional instrument about the longitudinal axis it is achieved that the strip is oriented parallel to the steady magnetic field or transversely to (preferably perpendicularly to) the steady magnetic field. When the strip extends parallel to the steady magnetic field, it causes hardly any or no influencing of the magnetic resonance image. When the strip extends more or less transversely to the steady magnetic field, it causes a given influencing of the magnetic resonance image. The influencing is dependent on the angle enclosed by the strip relative to the steady magnetic field. Influencing is maximum when the strip extends perpendicularly to the steady magnetic field. The indicator element may alternatively include a plurality of paramagnetic strips, said strips being arranged in such a manner that, when the interventional instrument is used during operation of the magnetic resonance imaging system, the strips are non-rotationally symmetrically arranged relative to the steady magnetic field.
The indicator element preferably includes a number of segments of different magnetic susceptibility. For example, the indicator element includes strips with segments in the form of parts of the strip of different materials with a different magnetic susceptibility. In another embodiment the segments are formed by separate parts of the strip whose respective dimensions differ in the direction transversely to the axis of the holder. The indicator element with a plurality of segments of different susceptibility causes a plurality of disturbances in the magnetic resonance image; these disturbances occur at distances from one another which correspond to the distances between the individual segments. Preferably, the separate segments of the indicator element are arranged at regular distances from one another. It is thus achieved that the respective segments cause disturbances in the magnetic resonance image which occur at regular distances from one another therein. A regular pattern of this kind, for example a row of disturbances at mutually equal distances, can be used as a distance scale in the magnetic resonance image. Using this distance scale, the interventional instrument can be readily displaced over a desired distance within the body of the patient. The interventional instrument can thus be accurately moved to a desired position while effectively avoiding unnecessary damaging of tissue.
A preferred embodiment of a magnetic resonance imaging system is arranged to perform a first type of magnetic resonance imaging sequence to generate magnetic resonance signals mainly relating to an interventional instrument which is introduced in an object to be examined and to perform a second type of magnetic resonance imaging sequence to generate magnetic resonance signals mainly relating to said body to be examined. Preferably the magnetic resonance imaging system is arranged to perform the MR imaging of the interventional instrument on the one hand and of the body into which the interventional instrument is introduced on the other hand on the basis of different magnetic resonance imaging sequences. Thus it is made possible to optimise both the imaging of the interventional instrument on the basis of magnetic resonance signals influenced or generated by the indicator element as well as the imaging of the tissue of the patient to be examined.